Behaviour of cobalt during sulphuric acid pressure leaching of a limonitic laterite

Abstract

Extraction of nickel and cobalt from lateritic ores by sulphuric acid pressure leaching techniques has become increasingly important in recent years. The above hydrometallurgical method has a low operational cost and is environmentally safe compared to the energy-intensive and air-polluting pyrometallurgical processes. High nickel and cobalt recovery of more than 90%, with a high selectivity due to the simultaneous iron and aluminium dissolution and precipitation, can be achieved. Experiments were carried out using batch pressure leaching techniques. A titanium autoclave equipped with acid injection and sample withdrawal units was employed. Conditions close to the industrial practice were tested; pulp density: 10–30%, acid to ore ratio: 0.15–0.35 and temperature ranging from 230 to 270 °C. Raw limonitic laterite and the evolution of the nature of solid products during leaching were thoroughly investigated using Transmission Electron Microscopy (TEM). Whereas nickel is predominately associated with the goethite phase, cobalt was found to exist only in nickel-rich manganese structures. During leaching, goethite dissolves liberating nickel while iron re-precipitates as dense hematite particles in solution by ex situ precipitation. Cobalt dissolves rapidly and remains in the aqueous phase. Manganese follows the cobalt dissolution pattern, however at a much lower rate. In the end of the leaching tests, manganese particles get almost depleted of cobalt. The dissolution rate of cobalt during leaching was insensitive to the increase of temperature in the above tested range. However, increasing the agitation rate of the slurry led to the increase of its dissolution rate. The above findings, along with the TEM photos and the respective mineralogical analysis, indicate that film diffusion is most possibly the rate-controlling step. The shrinking core model could be employed in order to describe cobalt dissolution kinetics.